Nondestructive testing of articles and materials has become an integral part of quality control in modern manufacturing industries. This type of testing enables on-line and intensive evaluation of the structural soundness of products. One of the most commonly used forms of nondestructive testing is radiographic images taken on industrial materials. Industrial X-rays have been used for many years in the testing of support beams used in the construction of buildings, bridges and the like. They are particularly useful in the evaluation of welds and in testing metal plates for minute flaws which could affect performance.
As industrial demands on materials become more stringent and the tolerance for flaws becomes reduced, more precise testing methods are required. In all imaging processes, including photography and radiography, there is an inherent limit in the resolution available through the process because of the physical elements used. In the practice of modern industrial X-ray procedures, the use of intensifying screens adds a further limit on the resolution available in radiographs. It has heretofore been generally accepted that the phosphor grains in intensifying screens and the screens themselves were the limiting factor in the graininess or resolution available in radiographs used in nondestructive testing (cf. Nondestructive Testing, 2d Ed. Warren J. McGonnagle, Science Publishers, 1971, pages 119-123, Radiography in Modern Industry, 3d Ed., Eastman Kodak, 1969, pages 34-38, and Physics of Industrial Radiology, R. Halmshaw, London, Heywood Books, 1966, pages 110 and 176). This limitation was believed to be a result of the fact that visible radiation emitted from the phosphor grain is spread out rather than projected in a linear path like the incident X-rays.
Silver halide photothermographic imaging materials, often referred to as `dry silver` compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and British Pat. No. 1,110,046 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver which is in catalytic proximity to the specks of the latent image.
Photothermographic emulsions, because of their relatively slow speed and coarse images, have generally been limited to high intensity machine exposures and have not been used with low intensity light exposure.